Random access procedure for latency reduction

ABSTRACT

Certain embodiments disclose a method in a network node. The method broadcasts a location of a time and/or frequency resource of a first Physical Random Access Channel (PRACH). The first PRACH has a static location. The method determines a location of a time and/or frequency resource for a second PRACH. The location of the second PRACH is determined dynamically. The method communicates downlink control information to a wireless device. The downlink control information indicates the location of the second PRACH. The method receives a random access attempt from the wireless device. The random access attempt is received via the first PRACH or the second PRACH. The method communicates a random access response to the wireless device.

This application is the National Stage of International Application No.PCT/IB2016/055280, which claims the benefit of U.S. ProvisionalApplication No. 62/085,958, filed Sep. 18, 2015, the disclosures ofwhich are fully incorporated herein by reference.

TECHNICAL FIELD

Certain embodiments of the present disclosure relate, in general, towireless communications and, more particularly, to random accessprocedures for latency reduction.

BACKGROUND

Packet data latency is a key performance metric in today's communicationsystems. Patent data latency is regularly measured by vendors,operators, and end-users (e.g., via speed test applications). Latency ismeasured throughout the lifetime of a radio access network system. Forexample, latency is measured when verifying a new software release orsystem component, when deploying a system, and after the system is putin commercial operation.

From the beginning, the long term evolution (LTE) radio accesstechnology was designed with low latency in mind. As a result, today LTEhas better packet data latency than previous generations of 3rdGeneration Partnership Project (3GPP) radio access technologies. A widerange of end-users recognize LTE as a system that provides faster accessto internet and lower data latencies than previous generations of mobileradio technologies.

Since the introduction of LTE in 2009, several improvements have beendeveloped, such as Carrier Aggregation (CA), 8×8 multiple-inputmultiple-output (MIMO) operation, and so on. The main target of theimprovements has been increasing the maximum data rates of the system.To get the full benefit of these data rate enhancements, enhancements toreduce latency should be an important part of the future evolution trackof LTE. An ongoing 3GPP study item aims to shorten the packet datalatency over the LTE air interface. One of the discussed options is toshorten the transmission time interval (TTI) length, which is currently14 Orthogonal Frequency Division Multiplexing (OFDM) symbols.

Certain embodiments of the present disclosure relate to random accessprocedures for latency reduction. As background, FIGS. 1-2 illustrateprior random access procedures in LTE. During initial access, a userequipment (UE) seeks access to the network in order to register andcommence services. The random access procedure serves as an uplinkcontrol procedure to enable the UE to access the network and acquireproper uplink timing (synchronize uplink). Since the initial accessattempt cannot be scheduled by the network, the initial random accessprocedure is by definition contention based. Collisions may occur and anappropriate contention-resolution scheme should be implemented.Including user data on the contention-based uplink is not spectrallyefficient due to the need for guard periods and retransmissions.Therefore, the LTE specification separates the transmission of therandom access burst (preamble), whose purpose is to obtain uplinksynchronization, from the transmission of user data.

Other reasons for initiating the random access procedure, beyond initialnetwork access or establishing a radio link (i.e., moving radio resourcecontrol (RRC) state from RRC_IDLE to RRC_CONNECTED) include performinghandover to establish uplink synchronization to a new cell, establishinguplink synchronization when the UE needs to transmit in the uplink(e.g., data or hybrid automatic repeat request (HARQ) feedback) when ithas lost uplink synchronization while in RRC_CONNECTED, and when noscheduling request resources have been configured on the Physical UplinkControl Channel (PUCCH) and the UE wants to transmit data in the uplink.

When uplink data arrives and the UE wants to transmit, it needs to be inRRC_CONNECTED mode, have its uplink synchronized (assigned MAC timealignment timer has not expired), and have scheduling request resourcesconfigured. If any of these requirements is not met, the UE initiatesthe random access procedure. The goal of the procedure is to acquireproper uplink timing in order for the UE to be able to send uplink data.

FIGS. 1-2 outline basic random access procedures. The figures illustratemessages communicated between a UE and a network node, such as anenhanced Node B or “eNB.” FIG. 1 illustrates a contention based randomaccess procedure in the case of initial access. At step 10, the UE sendsa random access preamble to the network node. In LTE, the random accesspreambles are transmitted over the Physical Random Access Channel(PRACH). The transmission of preambles is limited to certain time andfrequency resources. The time and frequency resources are configured byupper layers (in system information). For Frequency Division Duplex(FDD—frame structure format 1), the PRACH frequency can currently varyfrom every subframe to once in every other radio frame (i.e., once inevery 20 ms).

The PRACH resource has bandwidth corresponding to 6 physical resourceblocks. The length of the PRACH preamble in time depends on the preambleformat being used. For example, the basic format 0 fits into onesubframe (1 ms) and can be used in cell sizes up to 15 km. For FDD, onlyone random access region per subframe can be configured. There are 64different preamble sequences available in each cell. The preambles canbe divided into (two) subsets, and the UE selects one sequence from onesubset uniformly at random before performing the preamble transmission.The configuration of the PRACH resources in a cell is done by RRCprotocol, and the configuration is the same for all UEs in a cell.

At step 12, the network node sends the UE a random access response. InLTE, the random access response can be sent using the Physical DownlinkShared Channel (PDSCH). The random access response includes an initialassignment of uplink resources. At step 14, the UE sends the networknode an RRC Connection Request. The message is sent using the uplinkresources assigned by the network node in step 12. The message requeststo establish a connection at the radio resource control (RRC) layer. InLTE, the RRC Connection Request can be sent on the Physical UplinkShared Channel (PUSCH). At step 16, the network node sends the UE an RRCConnection Setup message in order to establish the RRC connection.

FIG. 2 illustrates an example of contention free random access in thecase of initial access. At step 20, the network node sends a randomaccess preamble assignment to the UE. Assignment of the random accesspreamble by the network node allows the network node to coordinate theallocation of random access preambles among a number of UEs so that therandom access procedure can be contention-free. At step 22, the UE sendsthe network node the random access preamble that was assigned to the UEin step 20. For simplicity, FIG. 2 illustrates an example in which theUE sends the random access preamble to the same network node thatassigned the random access preamble. However, it is possible for the UEto send the random access preamble to a different network node (i.e., anetwork node other than the one that assigned the preamble), forexample, in the case of a handover. At step 24, the network node sendsthe random access response. As described with respect to FIG. 1, the UEand network node may establish an RRC Connection after the network nodehas sent the random access response.

Random access procedures can introduce latencies. For example, in thecontention-based random access procedure described with respect to FIG.1, the UE may have to wait for a PRACH opportunity before sending apreamble. The wait depends on the periodicity of the PRACH. As anexample, the wait may be 0.5 TTI. Preamble transmission may require 1TTI. The network node receives the preamble and processes the preamble.Processing may introduce a delay that depends on the implementation ofthe network node. The delay may be on the order of 3 TTI. The networknode then sends the random access response to the UE via the PDSCH. TheUE listens during the random access response window and receives theresponse after 1 TTI. The UE decodes the uplink grant and performs L1encoding of uplink data. The UE processing delay may be on the order of5 TTIs. The UE sends uplink data to the network node, which may requirean additional 1 TTI. Thus, the total time for the random accessprocedure in the example is 11.5 TTIs.

SUMMARY

Embodiments of the present disclosure provide solutions to problemsassociated with existing random access procedures. One problem withexisting random access procedures it that the preamble length does notallow a UE to maximize the benefits of shorter TTI transmissions whenthe UE is out-of-synch. Additionally, the UE may need to wait until thenext PRACH opportunity before it can send the random access preamble. Itis possible in current specifications to have PRACH every subframe, butin practice this is typically not used as it is very resourceinefficient to have dedicated PRACH for every subframe.

The proposed solutions include defining and scheduling uplink resourcesfor a new physical random access channel, referred to herein as ashortened-PRACH or sPRACH, and defining procedures for using the sPRACH.For example, the present disclosure includes procedures that wirelessdevices/UEs can use when transmitting random access preambles via thesPRACH and procedures that network nodes can use when receiving randomaccess preambles via the sPRACH. In certain embodiments, dynamicscheduling initiated by the medium access control (MAC) layer may beused. Certain embodiments of the disclosure make use of the existing LTEresource grid so as not to affect legacy devices or devices not usinglatency improvements.

The present disclosure proposes both contention-free andcontention-based procedures for random access targeted for low latencyoperation. In certain embodiments, the sPRACH is scheduled in downlinkcontrol information (DCI). In certain embodiments, the sPRACH isconfigured with a preamble sequence that is shorter than a legacypreamble sequence. As an example, the sPRACH preamble sequence isshorter than the 800 microseconds length PRACH preamble that, togetherwith cyclic prefix and guard time, occupies 1 millisecond (i.e., 14 OFDMsymbols) in legacy LTE specifications. The shorter preamble sequence maymaximize the benefits of shorter TTI transmissions. The shorter preamblecosts in terms of bandwidth or the number of accessible preamblesequences with respect to the legacy PRACH. Out-of-synch UEs could usethe sPRACH to achieve shorter latency between when the UE receives datathat is supposed to be sent to the network node via the uplink and whenthe UE is able to send the received data to the network node. Likewise,in the contention-based alternative, time spent for the whole randomaccess procedure will be shorter. Scheduling of sPRACH differs fromlegacy (static) PRACH and can be controlled by the MAC layer, forexample, by including the sPRACH scheduling within theDCI/semi-persistent scheduling (SPS) grant. The proposed solutions workboth if the TTI length is reduced from current 14 OFDM symbols or if thelegacy TTI length is kept within legacy subframes.

Certain embodiments disclose a method in a network node. The methodbroadcasts a location of a time and/or frequency resource of a firstPhysical Random Access Channel (PRACH). The first PRACH has a staticlocation. The method determines a location of a time and/or frequencyresource for a second PRACH. The location of the second PRACH isdetermined dynamically. The method communicates downlink controlinformation to a wireless device. The downlink control informationindicates the location of the second PRACH. The method receives a randomaccess attempt from the wireless device. The random access attempt isreceived via the first PRACH or the second PRACH. The methodcommunicates a random access response to the wireless device.

Certain embodiments disclose a network node comprising memory and one ormore processors. The network node is operable to broadcast a location ofa time and/or frequency resource of a first Physical Random AccessChannel (PRACH). The first PRACH has a static location. The network nodeis operable to determine a location of a time and/or frequency resourcefor a second PRACH. The location of the second PRACH determineddynamically. The network node is operable to communicate downlinkcontrol information to a wireless device. The downlink controlinformation indicates the location of the second PRACH. The network nodeis operable to receive a random access attempt from the wireless device.The random access attempt is received via the first PRACH or the secondPRACH. The network node is operable to communicate a random accessresponse to the wireless device.

In certain embodiments, the time and/or frequency resource of the firstPRACH is defined according to a legacy 3GPP standard. In certainembodiments, the second PRACH is shorter than the first PRACH. Incertain embodiments, the first PRACH uses at least a portion of a samesubframe as the second PRACH and the first PRACH uses differentsubcarriers than the second PRACH. In certain embodiments, a singlesubframe comprises both (a) the downlink control information thatindicates the location of the second PRACH, and (b) the second PRACH. Incertain embodiments, the downlink control information implicitly orexplicitly indicates a preamble sequence to be used by the wirelessdevice when sending random access attempts via the second PRACH. Incertain embodiments, the location of the second PRACH is determineddynamically based on granting a semi persistent uplink grant for thesecond PRACH resource to the wireless device. In certain embodiments,the method further comprises broadcasting a message indicating that thenetwork node supports the second PRACH.

Certain embodiments disclose a method in a wireless device. The methodreceives a location of a time and/or frequency resource of a firstPhysical Random Access Channel (PRACH) from a network node. The firstPRACH has a static location. The method receives downlink controlinformation from the network node. The downlink control informationindicates the location of a second PRACH. The second PRACH has a dynamiclocation. The method communicates a random access attempt via the secondPRACH.

In certain embodiments, the method further comprises receiving a messageindicating that the network node supports the second PRACH and, inresponse, monitoring the downlink control information to determine thelocation of the second PRACH.

In certain embodiments, the method further comprises communicating arandom access attempt via the first PRACH in response to determiningthat the random access attempt via the second PRACH is unsuccessful.

In certain embodiments, the method further comprises communicating arandom access attempt via the first PRACH in response to determiningthat there is no second PRACH.

Certain embodiments disclose a wireless device comprising a memory andone or more processors. The wireless device is operable to receive alocation of a time and/or frequency resource of a first Physical RandomAccess Channel (PRACH) from a network node. The first PRACH has a staticlocation. The wireless device is operable to receive downlink controlinformation from the network node. The downlink control informationindicates the location of a second PRACH. The second PRACH has a dynamiclocation. The wireless device is operable to communicate a random accessattempt via the second PRACH.

In certain embodiments, the wireless device is further operable toreceive a message indicating that the network node supports the secondPRACH and, in response, monitor the downlink control information todetermine the location of the second PRACH.

In certain embodiments, the wireless device is further operable tocommunicate a random access attempt via the first PRACH in response todetermining that the random access attempt via the second PRACH isunsuccessful.

In certain embodiments, the wireless device is further operable tocommunicate a random access attempt via the first PRACH in response todetermining that there is no second PRACH.

In certain embodiments, the time and/or frequency resource of the firstPRACH is defined according to a legacy 3GPP standard. In certainembodiments, the second PRACH is shorter than the first PRACH. Incertain embodiments, the first PRACH uses at least a portion of a samesubframe as the second PRACH and the first PRACH uses differentsubcarriers than the second PRACH. In certain embodiments, a singlesubframe comprises both (a) the downlink control information thatindicates the location of the second PRACH, and (b) the second PRACH. Incertain embodiments, the downlink control information implicitly orexplicitly indicates a preamble sequence to be used by the wirelessdevice when sending random access attempts via the second PRACH. Incertain embodiments, receiving the downlink control informationindicating the location of the second PRACH implicitly or explicitlyindicates that the wireless device is to override the first PRACH.

Certain embodiments of the present disclosure may provide one or moretechnical advantages. As an example, certain embodiments shorten thelatency associated with legacy random access procedures. As anotherexample, certain embodiments configure contention-free access for alarger number of UEs compared to legacy, e.g., due to a certainfrequency allocation, such as center-6 physical resource blocks (PRBs).As another example, low-latency UEs (UEs using the sPRACH) do notincrease the load of the legacy PRACH channel or processing of legacyRACH procedure. As another example, a dedicated random access region forthe sPRACH avoids traffic mixing and avoids the need for a completedesign change. As another example, certain embodiments allow forresource use flexibility and load balancing. Flexible scheduling can becontrolled by the MAC layer. As another example, shorter preambleformats can be configured with reduced TTI lengths or with legacy TTIlengths. Certain embodiments may have all, some, or none of theseadvantages. Other advantages will be apparent to persons of ordinaryskill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example message flow for a contention based randomaccess procedure in the case of initial access, as known in the priorart.

FIG. 2 illustrates an example message flow for a of contention freerandom access in the case of initial access, as known in the prior art.

FIG. 3A illustrates an example of a flowchart for a contention-freerandom access procedure, in accordance with certain embodiments of thepresent disclosure.

FIG. 3B illustrates an example of a message flow for a contention-freerandom access procedure, in accordance with certain embodiments of thepresent disclosure.

FIG. 4A illustrates an example of a resource grid configured with alow-latency physical random access channel, in accordance with certainembodiments of the present disclosure.

FIG. 4B illustrates an example of a resource grid configured with alow-latency physical random access channel, in accordance with certainembodiments of the present disclosure.

FIG. 5 illustrates an example of a resource grid configured with alow-latency physical random access channel, in accordance with certainembodiments of the present disclosure.

FIG. 6 illustrates an example of a resource grid configured with alow-latency physical random access channel, in accordance with certainembodiments of the present disclosure.

FIG. 7 illustrates an example of a message flow for random access, inaccordance with certain embodiments of the present disclosure.

FIG. 8 illustrates an example block diagram of a wireless communicationnetwork, in accordance with certain embodiments of the presentdisclosure.

FIG. 9 illustrates an example block diagram of a wireless device, inaccordance with certain embodiments of the present disclosure.

FIG. 10 illustrates an example block diagram of a network node, inaccordance with certain embodiments of the present disclosure.

FIG. 11 illustrates an example block diagram of components of a wirelessdevice, in accordance with certain embodiments of the presentdisclosure.

FIG. 12 illustrates an example block diagram of components of a networknode, in accordance with certain embodiments of the present disclosure.

FIG. 13 illustrates an example of a message flow for random access, inaccordance with certain embodiments of the present disclosure.

FIG. 14 illustrates an example of a message flow for random access, inaccordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

Wireless networks use random access procedures to initiate connectionsbetween wireless devices 110 and network nodes 120. The random accessprocedures used in legacy wireless networks tend to introduce latencies.For example, latencies can be introduced when waiting for an opportunityto transmit random access related messages, when transmitting the randomaccess related messages over the wireless interface, and/or whenprocessing the random access related messages. The present disclosureproposes solutions that may reduce latencies associated with randomaccess procedures. The proposed solutions include defining andscheduling uplink resources for a new physical random access channel,referred to herein as the sPRACH, and defining procedures for using thesPRACH. For example, the present disclosure includes procedures thatwireless devices 110 (which are interchangeably referred to herein asUEs) can use when transmitting random access preambles via the sPRACHand procedures that network nodes 120 can use when receiving randomaccess preambles via the sPRACH.

In an embodiment, the sPRACH resources are defined in the LTE resourcegrid in a way that legacy UEs are not affected. For example, legacy UEsare not scheduled on the resources reserved for the sPRACH. The sPRACHuses a shorter preamble sequence than the legacy preamble sequence. Asexamples, FIGS. 4-5 described below illustrate embodiments in which 4OFDM symbols are reserved for the sPRACH (as compared to the 14 OFDMsymbols reserved for the legacy preamble sequence), and FIG. 6 describedbelow illustrates an embodiment in which 3 OFDM symbols are reserved forthe sPRACH (as compared to the 14 OFDM symbols reserved the legacypreamble sequence). The shorter preamble sequences decrease latency. ThesPRACH can be scheduled in various ways depending on factors such aswhether the random access is contention-based or contention-free.

In one embodiment, shorter preamble sequences make it possible tomultiplex a number of UEs in the same legacy TTI (1 ms) both in the timeand code domains. Depending on the length of the preamble, it could alsobe possible to have the preamble sent within the same legacy TTI that isused to send and/or receive subsequent messages, such as the randomaccess response message. As an example, assume a 200 μs preamble N×100μs processing M×100 μs random access reception where (2+N+M)<=10. Thiswould require defining sPUSCH and downlink resources in a compatible way(see e.g., FIG. 4A).

Certain embodiments are described using shortened TTI versions of thecurrent LTE physical channels, e.g., shortened-Physical Downlink ControlChannel (sPDCCH), shortened-Physical Downlink Shared Channel (sPDSCH),and shortened-Physical Uplink Shared Channel (sPUSCH). The exact name ofthese channels may be different, but the idea is that these channels aredefined and used for shorter TTI than the current 14 OFDM symbols. Thesolutions would work if future 3GPP releases introduce shorter TTIs (seee.g., FIG. 6), and also if the legacy TTI structure is kept by definingthe sPRACH inside existing TTIs in the LTE resource grid (see e.g.,FIGS. 4-5).

FIGS. 3A-3B illustrates examples of contention-free random accessprocedures, in accordance with certain embodiments of the presentdisclosure. In general, the contention-free random access alternativeschedules the sPRACH in downlink control information (DCI) dynamically.This means wireless device 110 listens to the control channel (e.g.,PDCCH/EPDCCH/sPDCCH) and decodes DCI, which further indicates availablesPRACH resources. Network node 120 also instructs wireless device 110 touse a specific preamble so that a separate UE identity does not need tobe sent during the random access procedure.

In one embodiment, the used sPRACH resources and/or preamble sequenceare derived through the used Cell Radio Network Temporary Identifier(C-RNTI) or similar identifier used to address downlink controlinformation to specific wireless devices 110. The derivation can be doneby using a function that maps the used identifier (e.g., C-RNTI) to thepreamble/sPRACH resources. That is, the used preamble/sPRACHresources=f(C-RNTI).

In the flow chart illustrated in FIG. 3A, at step 30, network node 120broadcasts a first Physical Random Access Channel (sPRACH) configurationor directly configures wireless device 110 to use the sPRACH. At step32, network node 120 sends downlink control information (DCI) includinga wireless device identifier (e.g., C-RNTI). For example, the DCI mayinclude a cyclic redundancy check (CRC) that has been scrambled with theC-RNTI. In one alternative, the procedure continues to step 34 (omittingstep 36), and wireless device 110 checks for its C-RNTI, reads the DCI,and sends a preamble sequence on the sPRACH resources indicated in theDCI. In another alternative, the procedure continues from step 32 tostep 36 (omitting step 34), and wireless device 110 checks for itsC-RNTI, reads the DCI, and implicitly derives the used preamble and/orsPRACH resources through the used C-RNTI.

In one embodiment the sPRACH resources which are used are mapped to thesame resources used for sPUSCH, when these sPUSCH resources are free(not scheduled).

FIG. 3B illustrates example steps for a contention-free procedure. Themethod begins with network node 120 (e.g., an eNB) detecting the arrivalof downlink data for wireless device 110. At step 40, network node 120instructs wireless device 110 to select a specific random accesspreamble and gives the sPRACH configuration to be used by the wirelessdevice 110. The signaling is done via the MAC layer using DCI over PDCCHor EPDCCH or sPDCCH (or any similar downlink control channel defined forthis purpose). In one embodiment, embodiment A, some of the legacy DCIformats are reused for this purpose, for example DCI format 1A. Inanother embodiment, embodiment B, a new DCI format is defined containingat least information on what preamble sequence to use. Additionally, thesPRACH resource to be used can be included. Additionally, a semipersistent grant might be given, which specifies the maximum duration ofthe grant.

A MAC entity of wireless device 110 receives the DCI over the PDCCH (orEPDCCH or sPDCCH), and at step 42 wireless device 110 transmits thesignaled preamble using the sPRACH channel. The used preamble and/orsPRACH resource to be used can be signaled explicitly in DCI or derivedimplicitly, e.g., from the used C-RNTI.

At step 44, network node 120 answers by sending a Random Access Response(RAR) to the UE, containing at least timing alignment information. Incertain embodiments, this RAR is carried by sPDSCH in order to reducelatency. (Optionally, the RAR can be carried by PDSCH).

In an example embodiment where the DCI is sent over PDCCH, the actualsPRACH channel is located in the same subframe with the correspondingPDCCH. This can be done when the preambles are short enough so they willfit in the same subframe. In this case the preamble sequence lengthwould be 11 OFDM symbols at maximum, preferably less to account fordistant-dependent time uncertainty. See FIG. 5 for an example of theresource grid for preamble length of 4 OFDM symbols.

Similarly, in alternative embodiments, if other types of controlchannels are used, the sPRACH resources could be located in the samesubframe if the control channel is located earlier in time domaincompared to sPRACH resources, so that the DCI can be decoded before UEstarts sending the preamble sequence.

While the particular embodiments are described above in terms ofparticular methods, as indicated above, these methods may be performedby particular network nodes (e.g., eNB, UE, etc.). These nodes maycomprise a processor and a memory containing computer-executableinstructions. When these instructions are executed by the processor, thenetwork node is thereby operative to perform the steps discussed above.These network nodes are discussed in more detail below with regard toFIGS. 8-12 below.

The preceding examples have been described in the context ofcontention-free random access. In addition, or in the alternative,certain embodiments may support contention-based random access. In thecontention-based random access alternative, one or more wireless devices110 receive a grant for using the sPRACH resources, and the wirelessdevices 110 transmit if they need. This can result in contention ifseveral wireless devices 110 select the same preamble to be transmittedover the same resources at the same time. However, different sPRACHpreambles from different users can be detected independently by the useof different root preamble sequences or by different cyclic shifts ofthe root preamble sequences.

In one embodiment, semi-persistent scheduling (SPS) or a similar schemeis used to schedule the used sPRACH resources, e.g., within the SPSgrant. This scheduling is done by sending a persistent “grant” forwireless devices 110 supporting and using the sPRACH feature. The grantshould include at least the location of the used resources (time andfrequency) and the periodicity of the sPRACH if it is not scheduledcontinuously. When the sPRACH resources are not expected to be usedanymore the grant can be released.

If contention is detected, it can be resolved using similar mechanismcurrently used during the random access procedure.

While the particular embodiments are described above in terms ofparticular methods, as indicated above, these methods may be performedby particular network nodes (e.g., eNB, UE, etc.). These nodes maycomprise a processor and a memory containing computer-executableinstructions. When these instructions are executed by the processor, thenetwork node is thereby operative to perform the steps discussed above.These network nodes are discussed in more detail below with regard toFIGS. 8-12 below.

FIGS. 4-6 illustrate examples of resource grids that can be configuredto include the sPRACH. In the contention-free case, the sPRACH isscheduled dynamically and thus there is no need for a staticconfiguration (other than network node 120 knowing that wireless device110 supports using sPRACH). In another alternative, the RRC protocol isused to configure a semi-static set of possible sPRACH resources, wherethe specific resources used are signaled to wireless device 110. Theconfiguration can be broadcasted in system information or sent towireless devices 110 using dedicated signaling, as in legacy operation.Similarly, the available set of preambles, if not fixed in thespecification, can be signaled using dedicated RRC signaling oralternatively in broadcasted system information.

In the contention-free case, the sPRACH could be statically configured(as PRACH today). However, more dynamic scheduling may be preferable.SPS or a similar scheme could be used for more dynamic reservation andrelease of the sPRACH resources.

In one embodiment, the dynamic sPRACH configuration by a new DCI formatdepends on a semi persistent uplink grant. Here, the sPRACH is forexample configured to use the same frequency allocation as in the SPS,but can be triggered to be transmitted at a specific time by a dynamicgrant.

FIG. 4A illustrates an LTE uplink resource grid in which the horizontaldimension is labeled by OFDM symbols and the vertical dimension islabeled by physical resource blocks (PRBs). One full (legacy) PRB-pairwould map to one row and OFDM symbols 1-14. The resource grid consistsof two resource block pairs in time domain (a first set of OFDM symbols1-14 followed by a second set of OFDM symbols 1-14) and 20 resourceblocks in frequency domain (PRBs 1-20). In FIG. 4A, “PU” refers to PUCCHand “L” refers to the legacy PRACH channel which in this example is onthe first PRB-pair (subframe) and the periodicity is larger than 1. ThesPRACH can be scheduled within the uplink resource grid. In FIG. 4A, “S”refers to sPRACH, which in this example is scheduled over 6 PRBs infrequency. In the example, the frequency location of the sPRACH (PRBs4-9) is not the same as the frequency location of the PRACH (PRBs 8-13).In the example, the sPRACH uses 4 OFDM symbols (OFDM Sym 1-4 in thesecond set of symbols), which is shorter than the 14 symbols used by thelegacy PRACH channel (OFDM Sym 1-14 in the first set of symbols). Thestriped area “---” after sPRACH could either contain further sPRACHresources/sequences (e.g., for longer preambles) or some other uplinkresources for wireless devices 110 supporting short TTIs (such as sPUSCHresources).

FIG. 4B is generally similar to FIG. 4A, however, FIG. 4B illustrates anexample in which the sPRACH (S) uses at least a portion of the samesubframe as the legacy PRACH (L) (e.g., both use OFDM Sym 1-4 in thefirst set of symbols) and the sPRACH uses one or more differentsubcarriers than the legacy PRACH (e.g., the sPRACH uses PRBs 3-8,whereas the legacy PRACH uses different PRBs 9-14).

As described above, it could also be possible to read the downlinkcontrol channel during the first OFDM symbols in a legacy TTI structure,so that the actual sPRACH channel would be later (in the same legacysubframe), enabling fast sending of the preambles immediately after thereceived downlink control information. FIG. 5 illustrates an examplewhere the preamble over sPRACH is sent after reading the DCI in the useddownlink control channel, assumed in this example to be located in thefirst 3 OFDM symbols of a TTI. FIG. 6 shows a further alternative withshorter TTI length. FIG. 6 illustrates an example of shorter TTI lengthsof 4 OFDM symbols. The symbol before the sPRACH channel (shown as “X” inthe figure) is used to receive and decode the DCI on a downlink controlchannel, and the remaining 3 OFDM symbols of the TTI are spent in thepreamble transmission over sPRACH.

FIG. 7 illustrates an example of a message flow, in accordance withcertain embodiments of the present disclosure. At step 700, network node120 broadcasts a location of a time and/or frequency resource of a firstPhysical Random Access Channel (PRACH). The first PRACH has a staticlocation. For example, in certain embodiments the time and/or frequencyresource of the first PRACH is defined according to a legacy 3GPPstandard. This may allow legacy wireless devices (i.e., devices that donot support the second PRACH) to continue to access the network via thefirst PRACH using legacy procedures.

Optionally, network node 120 broadcasts a message at step 702 indicatingthat the network node supports a second PRACH. Wireless device 110 mayreceive the message and, in response, monitor downlink controlinformation to determine the location of the second PRACH. At step 704,network node 120 determines a location of a time and/or frequencyresource for the second PRACH. The location of the second PRACH isdetermined dynamically. For example, in certain embodiments the locationof the second PRACH is determined dynamically based on granting a semipersistent uplink grant for the second PRACH resource to the wirelessdevice. In another example, the location of second PRACH is based on aplacement of transmissions from other UEs. For example, the other UEsmight have the benefit of a large continuous frequency allocation suchthat the sPRACH should be placed on the edge of the spectrum. In yetanother example, the sPRACH is placed on other resources as compared toplacements of sPRACH by other network nodes. In this way, theinterference, or risk for colliding PRACH preambles, is reduced. FIGS.4-6 illustrate resource grids showing example locations of the firstPRACH (labeled “L”) and the second PRACH (labeled “S”). The examplelocations include time resources (e.g., one or more OFDM symbols) andfrequency resources (e.g., one or more PRBs). In certain embodiments,the second PRACH is shorter than the first PRACH. As an example, FIGS.4-5 each use 14 OFDM symbols for the first PRACH and 4 OFDM symbols forthe second PRACH. Using a shorter PRACH may allow for latency reductionduring random access procedures via the second PRACH.

In certain embodiments, the first PRACH uses at least a portion of asame subframe as the second PRACH, and the first PRACH uses one or moredifferent subcarriers than the second PRACH (see e.g., FIG. 4B). Incertain embodiments, a single subframe comprises both (a) the downlinkcontrol information that indicates the location of the second PRACH, and(b) the second PRACH. For example, FIG. 5 illustrates an embodiment inwhich the downlink control information that indicates the location ofthe second PRACH may be located in the second OFDM symbols 1, 2, and 3(marked “---”) and the second PRACH may be located in the second OFDMsymbols 4, 5, 6, and 7 (marked “S”).

At step 706, network node 120 communicates downlink control informationto wireless device 110. The downlink control information indicates thelocation of the second PRACH. In certain embodiments, the downlinkcontrol information implicitly or explicitly indicates the preamblesequence to be used by the wireless device when sending random accessattempts via the second PRACH. In certain embodiments, the downlinkcontrol information implicitly or explicitly indicates that wirelessdevice 110 is to override the first PRACH. For example, wireless device110 may prioritize the second PRACH such that random access attempts arecommunicated via the second PRACH unless a determination is made to fallback to the first PRACH, such as discussed below with respect to FIGS.13-14.

At step 708, wireless device 110 communicates a random access attempt tonetwork node 120. In certain embodiments, the random access attemptcomprising a preamble sequence that wireless device 110 communicates viathe first PRACH or the second PRACH. At step 710, network node 120communicates a random access response to wireless device 110. At step712, wireless device 110 and network node 120 proceed with connecting anRRC connection.

Although the described solutions may be implemented in any appropriatetype of telecommunication system supporting any suitable communicationstandards and using any suitable components, particular embodiments ofthe described solutions may be implemented in an LTE network, such asthat illustrated in FIG. 8.

As shown in FIG. 8, the example network may include one or moreinstances of wireless devices 110 and network nodes 120. Examples ofwireless devices 110 include conventional user equipment (UEs) andmachine type communication (MTC)/machine-to-machine (M2M) UEs. Examplesof network nodes 120 include radio access nodes, such eNodeBs or otherbase stations capable of communicating with wireless devices 110. Thenetwork may also include any additional elements suitable to supportcommunication between wireless devices 110 or between a wireless device110 and another communication device (such as a landline telephone).Although the illustrated wireless devices 110 may representcommunication devices that include any suitable combination of hardwareand/or software, these wireless communication devices may, in particularembodiments, represent devices such as the example wireless device 110illustrated in greater detail by FIG. 9. Similarly, although theillustrated network node 120 may represent network nodes that includeany suitable combination of hardware and/or software, these nodes may,in particular embodiments, represent devices such as the example networknode 120 illustrated in greater detail by FIG. 10.

As shown in FIG. 9, the example wireless device 110 includes an antenna,a transceiver 112, a processor 114, and a memory 116. In particularembodiments, some or all of the functionality described above as beingprovided by UEs, MTC or M2M devices, and/or any other types of wirelesscommunication devices may be provided by the device processor 114executing instructions stored on a computer-readable medium, such as thememory 116 shown in FIG. 9. Alternative embodiments of wireless device110 may include additional components beyond those shown in FIG. 9 thatmay be responsible for providing certain aspects of the device'sfunctionality, including any of the functionality described above and/orany functionality necessary to support the solution described above.

As shown in FIG. 10, the example network node 120 includes an antenna, atransceiver 122, a processor 124, a memory 126, and a network interface128. In particular embodiments, some or all of the functionalitydescribed above as being provided by a radio access node, a basestation, a node B, an enhanced node B, a base station controller, aradio network controller, a relay station and/or any other type ofnetwork node may be provided by the node processor 124 executinginstructions stored on a computer-readable medium, such as the memory126 shown in FIG. 10. Alternative embodiments of network node 120 mayinclude additional components responsible for providing additionalfunctionality, including any of the functionality identified aboveand/or any functionality necessary to support the solution describedabove.

FIG. 11 illustrates an example of components that may be included inwireless device 110. The components include random access module 130 andconnection setup module 132. In certain embodiments, random accessmodule 130 receives (700) a location of a time and/or frequency resourceof a first Physical Random Access Channel (PRACH) from a network node(120), the first PRACH having a static location, and receives (706)downlink control information from the network node, the downlink controlinformation indicating the location of a second PRACH, the second PRACHhaving a dynamic location. Connection setup module 132 communicates arandom access attempt via the first PRACH and/or the second PRACH andfacilitates connection of the RRC. In certain embodiments, modules 130and/or 132 may be implemented by processor 114 of FIG. 9.

FIG. 12 illustrates an example of components that may be included innetwork node 120. The components include first PRACH module 140, secondPRACH module 142, and connection setup module 144. In certainembodiments, first PRACH module 140 broadcasts (700) a location of atime and/or frequency resource of a first Physical Random Access Channel(PRACH), the first PRACH having a static location. Second PRACH module142 determines (704) a location of a time and/or frequency resource fora second PRACH, the location of the second PRACH determined dynamically,and communicates (706) downlink control information to a wireless device110, the downlink control information indicating the location of thesecond PRACH. Connection setup module 144 receives (708) a random accessattempt from the wireless device via the first PRACH or the secondPRACH, communicates (710) a random access response to the wirelessdevice, and facilitates connection of the RRC. In certain embodiments,modules 140, 142, and/or 144 may be implemented by processor 124 of FIG.10.

FIG. 13 illustrates an example of a message flow, in accordance withcertain embodiments of the disclosure. At step 1300, network node 120broadcasts a location of a time and/or frequency resource of a firstPRACH. The first PRACH has a static location. Wireless device 110 maydetermine that there is no second PRACH, for example, if wireless device110 has not received any grant of a second PRACH or if a previous grantof a second PRACH has ended. If wireless device 110 determines to makean access attempt when there is no second PRACH, at step 1302 wirelessdevice 110 communicates the random access attempt via the first PRACH inresponse to determining that there is no second PRACH. At step 1304,network node 120 communicates a random access response to wirelessdevice 110. At step 1306, wireless device 110 and network node 120perform procedures for establishing an RRC connection. The RRCconnection may be disconnected (not shown) when the connection is nolonger needed.

At step 1308, network node 120 dynamically determines a location for asecond PRACH. At step 1310, network node 120 communicates downlinkcontrol information to wireless device 120. The downlink controlinformation indicates the location of the second PRACH and implicitly orexplicitly indicates that the wireless device is to override the firstPRACH. At step 1312, wireless device 110 communicates a random accessattempt via the second PRACH (rather than the first PRACH, which hasbeen overridden). At step 1314, network node 120 communicates a randomaccess response, and at step 1316 wireless device 110 and network node120 perform procedures for establishing an RRC connection. Wirelessdevice 110 may fall back to the first PRACH for subsequent random accessattempts, for example, in response to a determination that there is nosecond PRACH (e.g., if the grant of second PRACH ends) or in response toa determination that random access attempts using the second PRACH areunsuccessful, as discussed with respect to FIG. 14 below.

FIG. 14 illustrates an example of a message flow, in accordance withcertain embodiments of the disclosure. At step 1400, network node 120broadcasts a location of a time and/or frequency resource of a firstPRACH. The first PRACH has a static location. At step 1402, network node120 dynamically determines a location for a second PRACH. At step 1404,network node 120 communicates downlink control information to wirelessdevice 120. The downlink control information indicates the location ofthe second PRACH and implicitly or explicitly indicates that thewireless device is to override the first PRACH. At step 1406, wirelessdevice 110 communicates a random access attempt via the second PRACH(rather than the first PRACH, which has been overridden). At step 1408,wireless device 110 determines that the random access attempt via thesecond PRACH is unsuccessful. As an example, wireless device 110 maydetermine that the random access attempt is unsuccessful if a randomaccess response is not received from network node 120 within a certainnumber of attempts or within a pre-determined time period. In responseto determining that the random access attempt via the second PRACH isunsuccessful, wireless device 110 communicates a random access attemptvia the first PRACH at step 1410. At step 1412, network node 120communicates a random access response, and at step 1414 wireless device110 and network node 120 perform procedures for establishing an RRCconnection.

Reference has been made herein to various embodiments. However, a personskilled in the art would recognize numerous variations to the describedembodiments that would still fall within the scope of the claims. Anytwo or more embodiments described in this document may be combined inany suitable way with each other. Furthermore, the examples can beadapted in suitable radio access technologies.

The method embodiments described herein describes example methodsthrough method steps being performed in a certain order. However, it isrecognized that these sequences of events may take place in anotherorder without departing from the scope of the disclosed embodiments.Furthermore, some method steps may be performed in parallel even thoughthey have been described as being performed in sequence. Some methodsteps may be added or omitted without departing from the scope of thedisclosure.

In the same manner, it should be noted that in the description ofembodiments, the partition of functional blocks into particular units isby no means limiting. Contrarily, these partitions are merely examples.Functional blocks described herein as one unit may be split into two ormore units. In the same manner, functional blocks that are describedherein as being implemented as two or more units may be implemented as asingle unit without departing from the scope of the claims.

Hence, it should be understood that the details of the describedembodiments are merely for illustrative purpose and by no meanslimiting. Instead, all variations that fall within the range of theclaims are intended to be embraced therein.

The invention claimed is:
 1. A method in a network node, the methodcomprising: broadcasting a location of a time and/or frequency resourceof a first Physical Random Access Channel (PRACH) determining a locationof a time and/or frequency resource for a second PRACH communicating toa wireless device, the location of the time and/or frequency resourcefor the second PRACH; receiving a random access attempt from thewireless device, the random access attempt received via the first PRACHor the second PRACH; and communicating a random access response to thewireless device; wherein the first PRACH and the second PRACH each havean associated preamble, and wherein the second PRACH preamble has adifferent length than the first PRACH preamble; and wherein a singlesubframe comprises both (a) downlink control information that indicatesthe location of the second PRACH, and (b) the second PRACH.
 2. Themethod of claim 1, wherein the time and/or frequency resource of thefirst PRACH is defined according to a legacy 3GPP standard.
 3. Themethod of claim 1, wherein the second PRACH preamble is shorter than thefirst PRACH preamble.
 4. The method of claim 1, wherein: the first PRACHuses at least a portion of a same subframe as the second PRACH; and thefirst PRACH uses different subcarriers than the second PRACH.
 5. Themethod of claim 1, further comprising: broadcasting a message indicatingthat the network node supports the second PRACH.
 6. The method of claim1, wherein downlink control information indicates a preamble sequence tobe used by the wireless device when sending random access attempts viathe second PRACH.
 7. The method of claim 1, wherein the location of thesecond PRACH is determined dynamically based on granting a semipersistent uplink grant for the second PRACH resource to the wirelessdevice.
 8. A network node comprising memory and one or more processors,the network node operable to: broadcast a location of a time and/orfrequency resource of a first Physical Random Access Channel (PRACH)determine a location of a time and/or frequency resource for a secondPRACH communicate to a wireless device, the location of the time and/orfrequency resource of the second PRACH; receive a random access attemptfrom the wireless device, the random access attempt received via thefirst PRACH or the second PRACH; and communicate a random accessresponse to the wireless device; wherein the first PRACH and the secondPRACH each have an associated preamble, and wherein the second PRACHpreamble has a different length than the first PRACH preamble; andwherein a single subframe comprises both (a) downlink controlinformation that indicates the location of the second PRACH, and (b) thesecond PRACH.
 9. The network node of claim 8, wherein the time and/orfrequency resource of the first PRACH is defined according to a legacy3GPP standard.
 10. The network node of claim 8, wherein the second PRACHpreamble is shorter than the first PRACH preamble.
 11. The network nodeof claim 8, wherein: the first PRACH uses at least a portion of a samesubframe as the second PRACH; and the first PRACH uses differentsubcarriers than the second PRACH.
 12. The network node of claim 8,further operable to: broadcast a message indicating that the networknode supports the second PRACH.
 13. The network node of claim 8, whereindownlink control information indicates a preamble sequence to be used bythe wireless device when sending random access attempts via the secondPRACH.
 14. The network node of claim 8, wherein the location of thesecond PRACH is determined dynamically based on granting a semipersistent uplink grant for the second PRACH resource to the wirelessdevice.
 15. A method in a wireless device, the method comprising:receiving a location of a time and/or frequency resource of a firstPhysical Random Access Channel (PRACH) from a network node; receiving alocation of a time and/or frequency resource of a second PRACH;communicating a random access attempt via the first PRACH or the secondPRACH; and in response to receiving a message indicating that thenetwork node supports the second PRACH, monitoring downlink controlinformation to determine the location of the second PRACH; wherein thefirst PRACH and the second PRACH each have an associated preamble, andwherein the second PRACH preamble has a different length than the firstPRACH preamble.
 16. The method of claim 15, wherein the time and/orfrequency resource of the first PRACH is defined according to a legacy3GPP standard.
 17. The method of claim 15, wherein the second PRACHpreamble is shorter than the first PRACH preamble.
 18. The method ofclaim 15, wherein: the first PRACH uses at least a portion of a samesubframe as the second PRACH; and the first PRACH uses differentsubcarriers than the second PRACH.
 19. The method of claim 15, wherein asingle subframe comprises both (a) downlink control information thatindicates the location of the second PRACH, and (b) the second PRACH.20. The method of claim 15, wherein downlink control informationindicates a preamble sequence to be used by the wireless device whensending random access attempts via the second PRACH.
 21. The method ofclaim 15, further comprising: communicating a random access attempt viathe first PRACH in response to determining that the random accessattempt via the second PRACH is unsuccessful.
 22. The method of claim15, further comprising: communicating a random access attempt via thefirst PRACH in response to determining that there is no second PRACH.23. The method of claim 15, further comprising receiving downlinkcontrol information indicating the location of the second PRACH andindicating that the wireless device is to override the first PRACH. 24.A wireless device comprising a memory and one or more processors, thewireless device operable to: receive a location of a time and/orfrequency resource of a first Physical Random Access Channel (PRACH)from a network node; a location of a time and/or frequency resource of asecond PRACH; communicate a random access attempt via the first PRACH orthe second PRACH; and in response to receiving a message indicating thatthe network node supports the second PRACH, monitor downlink controlinformation to determine the location of the second PRACH; wherein thefirst PRACH and the second PRACH each have an associated preamble, andwherein the second PRACH preamble has a different length than the firstPRACH preamble.
 25. The wireless device of claim 24, wherein the timeand/or frequency resource of the first PRACH is defined according to alegacy 3GPP standard.
 26. The wireless device of claim 24, wherein thesecond PRACH preamble is shorter than the first PRACH preamble.
 27. Thewireless device of claim 24, wherein: the first PRACH uses at least aportion of a same subframe as the second PRACH; and the first PRACH usesdifferent subcarriers than the second PRACH.
 28. The wireless device ofclaim 24, wherein a single subframe comprises both (a) downlink controlinformation that indicates the location of the second PRACH, and (b) thesecond PRACH.
 29. The wireless device of claim 24, wherein downlinkcontrol information indicates a preamble sequence to be used by thewireless device when sending random access attempts via the secondPRACH.
 30. The wireless device of claim 24, further operable to:communicate a random access attempt via the first PRACH in response todetermining that the random access attempt via the second PRACH isunsuccessful.
 31. The wireless device of claim 24, further operable to:communicate a random access attempt via the first PRACH in response todetermining that there is no second PRACH.
 32. The wireless device ofclaim 24, further comprising receiving downlink control informationindicating the location of the second PRACH and indicating that thewireless device is to override the first PRACH.